1. Field of the Invention
The present invention relates to an image blur preventing device, for preventing image blur generated for example by hand vibration.
2. Related Background Art
For image blur prevention, there have conventionally been proposed various devices based on a principle of detecting hand vibration and displacing a correcting lens according to the detected value, thereby correcting the variation of image in optical axis. More specifically, the vibration is detected by a vibration sensor such as an angular acceleration meter or an angular speedometer, then the angular displacement is calculated electrically or mechanically, based on a signal from the vibration sensor, and the correcting lens is driven by information corresponding to the angular displacement.
The correction with the above-mentioned correcting lens can be achieved, for example, by effecting a parallel shift of the optical axis in a part of the phototaking lens, tilting the optical axis, a combination of these two methods, or providing a variable-angle prism (VAP) in front of the phototaking optical system.
Among these methods, decentralization of a part of the phototaking lens is achieved in the following manner. A base member of a blur correction unit is connected to a link member movable in a direction perpendicular to the phototaking optical axis (for example in the pitch direction), and a correcting lens frame, supporting a correcting lens, is supported by the link member so as to be movable in a direction perpendicular to the above-mentioned direction (for example in the yaw direction), whereby the correcting lens frame is rendered movable, with respect to the base member, in the pitch or yaw direction perpendicular to the optical axis.
As the actuators for driving the correcting lens, the correcting lens frame is provided with voice coils respectively corresponding to the pitch and yaw directions perpendicular to the phototaking optical axis, and yokes and magnets corresponding to these voice coils are fixed on the base member.
Also for detecting the eccentric position of the correcting lens in the pitch and yaw directions, light-emitting elements (or light-receiving elements) are fixed on the correcting lens frame while corresponding light-receiving elements (or light-emitting elements) are fixed on the base member, and the position of the correcting lens is detected by the amount of light emitted by the light-emitting elements and received by the light-receiving elements, and precise position control of the correcting lens is achieved by information corresponding to the thus detected position.
Such a blur correcting device may be powered by the power source of the main camera body, or may have an exclusive power source in consideration of the power capacity or function of the camera system and in order to enable automatic focusing or diaphragm driving simultaneously with the blur correction.
The function of the blur correcting device may be initiated by an external start switch which powers the blur correcting control circuit, vibration sensor, correcting optical system drive circuit, etc., to activate the blur correcting operation, or by an external switch and a shutter releasing signal of the camera, namely when the shutter release button of the camera is depressed by a half stroke while the external switch is turned on. Also the blur correcting operation may be terminated by turning off the external switch, or, even while the external switch is turned on, by an exposure operation of the camera by a full-stroke depression of the shutter release button or by the lapse of a predetermined time after the half-stroke depression of the shutter release button is terminated. At such termination, the correcting optical system is driven to a position where the optical axis thereof coincides with that of the phototaking lens and is fixed by mechanical locking means before the power supply is cut off.
Also there is proposed, as disclosed in the U.S. Pat. No. 5,181,056, a configuration in which the locking means for the correcting optical system is rendered operable by the biasing force of a spring and the correcting optical system can be locked by releasing an engaging member with a limited amount of electric power. Also there is known a configuration having a separate back-up power source which, in case the power source for blur correction becomes exhausted below a predetermined voltage or is cut off by detachment of the battery, supplies electric power to release the above-mentioned engaging member, thereby locking the correcting optical system.
However, in conventional configurations which are so designed, at the termination of the blur correcting operation, to drive the correcting lens to a position where the optical axis thereof coincides with that of the phototaking lens and to fix the correcting lens with mechanical locking means before the power supply is cut off, if such locking means is composed of a magnetic device such as a plunger and if the operator erroneously replaces the battery during the blur correcting operation or if the battery becomes detached by an external shock, the correcting lens remains unlocked and movable within the lens barrel whereby a phototaking operation becomes impossible and eventually the device itself may be damaged.